In digital measuring technology, sampling an analog measuring signal with the aid of an A/D converter at a high sampling rate and then digitally processing it further is known. The sensor signal is usually low-pass filtered before sampling to comply with the sampling theorem. This ensures that no frequencies that are greater than one-half of the sampling frequency occur in the sensor signal. In some measuring systems, it is, however, not possible for technical reasons to use low-pass filters having a sufficiently low critical frequency. In this case, it is impossible to prevent aliasing from occurring at a downstream digital low-pass filter. This is explained in greater detail using the example of FIG. 1:
FIG. 1 shows current signal I of a current sensor which works according to the inductive measuring principle. The polarity of a sensor coil is periodically switched over in this case. During this switchover operation, it is impossible to register any valid current measuring values. The range in which no valid measuring values may be registered is labeled with the reference numeral 4, and the range in which the current sensor delivers valid measuring values is labeled with the reference numeral 3.
To reconstruct current signal I in invalid measuring ranges 4, it is known to approximate the missing measuring values via a linear equation, for example. A straight line 20 is drawn between the latest validly measured current value and the next validly measured current value, and a plurality of intermediate values 6 lying on this straight line 20 is calculated. In the figure, valid sampling values are labeled with the reference numeral 5 and reconstructed sampling values with reference numeral 6. Approximated signal 1 is subject to an error of differing magnitudes depending on the phase angle of the reconstructed values. Signal 1 is removed by downstream low-pass filtering and only the reconstruction error is preserved. Since this error is not constant but fluctuates with the phase angle of the reconstructed values, a low-frequency error signal is obtained at the output of the low-pass filter, which may be viewed as aliasing.